Vehicle roof liners are generally made of a laminar structure comprising a plurality of layers conformed to the shape of the corresponding vehicle roof. For example, a typical roof liner can comprise one layer (for example of fabric) on the exposed surface (i.e. the surface facing the interior of the vehicle), at least one intermediate reinforcement layer (normally made of fibreglass) and one structural layer (normally of semi-rigid, low density polyurethane foam or polyester). Generally, a paper layer is disposed on the hidden surface, made of a non-fabric polythene or the like, or nothing at all is disposed. Some or all of the intermediate layers (this is, those between the layer corresponding to the exposed surface and that corresponding to the hidden surface) can be duplicated, their order may be changed and additional layers may be added according to the specific properties desired for the liner. In addition, between the layers (or between some of the layers) an adhesive can be present to keep the layers joined to each other. For example, it is common to use two reinforcement layers of fibreglass, one on either side of the structural layer.
The laminate layers can be joined by an adhesive (such as by a thermosetting resin) applied on the structural layer. Document ES-A1-2157725 describes an example of a liner of this type. Another example of the state of the art is EP-A-0832787.
It has been found that in certain areas of this type of liners (such as in areas of low thickness and embeddings) it may be necessary to use a greater amount of fibreglass, at specific places, to reinforce the liner. These reinforced layers also tend to have poor acoustic properties (in the sense that they do not conform to the acoustic requirements for internal auto parts; the liners should be good noise absorbers). Therefore, it is considered desirable to obtain liners with good acoustic properties and a high, reasonably homogenous stiffness, while having a reasonable cost of production. That is, a good ratio between acoustic properties, stiffness and cost is searched for.
U.S. Pat. No. 4,840,832 describes a roof liner with a central core made of a mat of PET (polyethylene terephthalate) fibres on which a small foam layer is placed (optionally) to improve the appearance of the liner (as it corrects the small irregularities that can appear in the fibre mat), to improve the acoustic properties and provide a soft feel to the user.
In this case, the foam layer employed has no structural function, as its function is strictly aesthetic, so that it could consist of the foam layer integrated in certain trilaminate exposed surface liners formed by the fabric (polyester), the foam layer (polyurethane) and a thermoplastic film. The function of the foam is mainly to provide a soft feel to the user.
U.S. Pat. No. 5,258,585 describes a laminate useful as a roof liner for vehicles. The laminate comprises a central core formed by a fibrous mat and a foam layer that can be joined by an adhesive layer activated by heat. In addition, the laminate has a fibrous reinforcement layer (polyester fibres and, optionally, fibreglass) and a coating layer (which in turn can consist of a foam layer and another fabric layer). Moreover, the laminate can comprise a polyethylene layer with orifices for improving the absorption of low frequency sounds.
The laminate has been designed considering its acoustic properties: the central core absorbs the high-frequency sounds and the polyethylene layer absorbs the low-frequency sounds.
To manufacture the laminate, the various layers comprising it are joined, the set is passed through an oven before pressing and it is then cold-formed. This is, it is a thermoplastic process: the adhesive used is a thermoplastic film, which allows heating before forming (to activate the adhesive) and then forming in a cold mould, which can be convenient.
U.S. Pat. No. 5,536,556 also describes an acoustic laminate for automobile liners.